The present invention pertains generally to apparatus and methods for molding plastic articles and more particularly to blow molding parisons which are formed in a molding with rotation process. The molding with rotation process is described in U.S. Pat. Nos. 3,307,726; 3,356,242; 3,371,387; 3,389,934; 3,401,427; 3,409,709; 3,500,503; and 4,083,568, which are specifically incorporated herein by reference and form a part of this disclosure for all that they teach.
Molding with rotation, such as disclosed in the U.S. patent application Ser. No. 684,673 filed on even date herewith by Roger Alan Hahn and Bruce Allen Moen entitled "Flash Suppression for Molding with Rotation" which is incorporated herein by reference and forms a part of this disclosure for all that it teaches, involves injection molding of a plastic article, such as a plastic parison formed between a rotating mandrel or core pin and a stationary mold cavity. The core pin is rotated during injection molding to impart a preferred orientation to long chains of molecules in the polymer, such as described in U.S. Pat. No. 3,307,726. The molded article thus possesses improved strength characteristics in the longitudinal and transverse directions due to the bi-axial orientation of the polymer chains in the parison. The parison, or preform, is then blow molded into the reform, i.e., the finished article such as a plastic bottle or plastic can.
Two processes exist for blow molding; the integrated and nonintegrated process. In accordance with the integrated process, the parison is formed by injection molding as disclosed above and the parison is blown with air and expanded against a female mold member while it is still warm from the injection molding process, such as described in U.S. Pat. Nos. 3,752,625 and 3,819,314, which are incorporated herein, and form a part of this disclosure, for all that they teach. The integrated molding with rotation and blow molding process suffers from several disadvantages and limitations. A high degree of complexity and technical difficulty exists in molding with rotation in an integrated process. For example, a much higher pin temperature is required in the integrated process which increases the temperature of the flash seal, and consequently, adversely affects its ability to suppress flash. In the nonintegrated process of reheating and blow molding, much cooler pin temperatures are utilized and, consequently, much cooler temperatures exist in the flash seal which improves its ability to suppress flash.
Additionally, the integrated process does not provide sufficient isolation between the processes of molding with rotation and blow molding to provide a sufficient amount of independent and versatile control over the automated system. For example, it is necessary to provide a significant equilibration time for the parison to reach a temperature for blow molding. In the integrated process, the pin is held at a temperature at which parison is blow molded to ensure that no discontinuity in temperature exists as a result of contact with the core pin. This high temperature at which the core pin is held greatly increases equilibration time and can significantly reduces the operating speed of the automated process. Otherwise, the blow molding process can be operated at a higher parison temperature which results in a weaker quality article. In the nonintegrated process, the parison can be blow molded at a lower temperature, resulting in a stronger quality article. Moreover, the operational speed of the automated process need not be limited to provide the time for the temperature of the parison to equilibrate.
Furthermore, the reliability of an integrated system is greatly reduced by the complexity and technical difficulty of achieving a commercially viable automated integrated system. The technical requirement of providing a molding with rotation system in conjunction with a blow molding system which can operate at high speeds results in a device with low reliability and high cost.
The nonintegrated process involves molding with rotation and storage of the parisons produced in the molding with rotation process for later reheating and blow molding. The nonintegrated process provides a system which is less complex, provides sufficient isolation between the processes to blow mold at lower temperatures so as to provide stronger products, and allows the molding with rotation seal to operate at lower temperatures to more readily suppress flash. However, the prior art methods of heating parisons have been nonuniform and have resulted in a finished article of weaker quality. For example, conduction heating through a pin holding the parison has been utilized in prior art designs. In the integrated process, heat conduction between the pin and the parison results in sufficiently uniform heating because the parison is form fitted on the pin during the injection molding process. In the nonintegrated process, contact between the parison and the pin is not sufficiently consistent to provide uniform heating because the parison is removed from the core pin used for injection molding and placed on another pin for blow molding. The nonuniform temperature of the parison during the blow molding process results in a weaker quality article.